1. Field of the Invention
The present invention relates to a technique of recording dots on the surface of a printing medium with a dot recording head.
2. Discussion of the Background
Serial scan-type printers and drum scan-type printers are dot recording devices which record dots with a recording head while carrying out scans both in a main scanning direction and a sub-scanning direction. There is a technique called the xe2x80x9cinterlace schemexe2x80x9d, which is taught by U.S. Pat. No. 4,198,642 and Japanese Patent Laid-Open Gazette No. 53-2040, for improving the image quality of printers of this type, especially ink jet printers.
FIG. 26 is a diagram for explaining an example of the interlace scheme. In this specification, the following parameters are used to define a printing scheme.
N: Number of nozzles;
k: Nozzle pitch [dots];
s: Number of scan repeats;
D: Nozzle density [nozzles/inch];
L: Sub-scanning pitch [dots] or [inch];
w: Dot pitch [inch].
The number of nozzles N is the number of nozzles actually used to form dots. In the example of FIG. 26, N=3. The nozzle pitch k is the interval between the centers of the recording head nozzles expressed in units of the recorded image pitch (dot pitch w). In the example of FIG. 26, k=2. The number of scan repeats s is the number of main scans in which all dot positions on a main scanning line are serviced. In the example of FIG. 26, s=1, i.e., all dot positions on a main scanning line are serviced in a single main scan. When s is 2 or greater, the dots are formed intermittently in the main scanning direction. This will be explained in detail later. The nozzle density D (nozzle/inch) is the number of nozzles per inch in the nozzle array of the recording head. The sub-scanning pitch L (inch) is the distance moved in 1 sub-scan. The dot pitch w (inch) is the pitch of the dots in the recorded image. In general, it holds that w=1/(Dxc2x7k), k=1/(Dxc2x7w).
The circles containing two-digit numerals in FIG. 26 indicate dot recording positions. As indicated in the legend, the numeral on the left in each circle indicates the nozzle number and the numeral on the right indicates the recording order (the number of the main scan in which it was recorded).
The interlace scheme shown in FIG. 26 is characterized by the configuration of the nozzle array of the recording head and the sub-scanning method. Specifically, in the interlace scheme, the nozzle pitch k indicating the interval between the centers of adjacent nozzles is defined as an integer not smaller than 2, while the number of nozzles N and the nozzle pitch k are selected as integers which are relatively prime. Further, sub-scanning pitch L is set at a constant value given by N/(Dxc2x7k).
The interlace scheme makes irregularities in nozzle pitch and ink jetting Feature to thin out over the recorded image. Because of this, it improves image quality by mitigating the effect of any irregularity that may be present in the nozzle pitch, the jetting Feature and the like.
The xe2x80x9coverlap schemexe2x80x9d, also known as the xe2x80x9cmulti-scan schemexe2x80x9d, taught for example by Japanese Patent Laid-Open Gazette No. 3-207665 and Japanese Patent Publication Gazette No. 4-19030 is another technique used to improve image quality in color ink jet printers.
FIG. 27 is a diagram for explaining an example of the overlap scheme. In the overlap scheme, 8 nozzles are divided into 2 nozzle sets. The first nozzle set is made up of 4 nozzles having even nozzle numbers (left numeral in each circle) and the second nozzle set is made up of 4 nozzles having odd nozzle numbers. In each main scan, the nozzle sets are each intermittently driven to form dots in the main scanning direction once every (s) dots. Since s=2 in the example of FIG. 27, a dot is formed at every second dot position. The timing of the driving of the nozzle sets is controlled so that the each nozzle set forms dots at different positions from the other in the main scanning direction. In other words, as shown in FIG. 27, the recording positions of the nozzles of the first nozzle set (nozzles number 8, 6, 4, 2) and those of the nozzles of the second nozzle set (nozzles number 7, 5, 3, 1) are offset from each other by 1 dot in the main scanning direction. This kind of scanning is conducted multiple times with the nozzle driving times being offset between the nozzle sets during each main scan to form all dots on the main scanning lines.
In the overlap scheme, the nozzle pick k is set at an integer no less than 2, as in the interlace scheme. However, the number of nozzles N and the nozzle pitch k are not relatively prime, but the nozzle pitch k and the value N/s, which is obtained by dividing the number of nozzles N by the number of scan repeats s, are set at relatively prime integers instead.
In the overlap scheme, the dots of each main scanning line are not all recorded by the same nozzle but by multiple nozzles. Even when the nozzle characteristics (pitch, jetting characteristic etc.) are not completely uniform, therefore, enhanced image quality can be obtained because the characteristics of the individual nozzles is prevented from affecting the entire main scanning line.
As described above, a variety of dot recording schemes have so far been proposed. In the actual state, the image quality also depends upon the manufacturing error of the dot recording apparatus. A desired dot recording scheme may thus be different for individual dot recording apparatuses that have been manufactured according to an identical design. In the prior art technique, it is difficult to adopt a desired dot recording scheme suitable for each individual dot recording apparatus.
The present invention is made to solve the problem of the prior art, and an object of the present invention is to provide a technique that enables a desired dot recording scheme to be adopted for each individual dot recording apparatus.
In order to solve at least part of the above problems, there is provided a dot recording apparatus for recording dots on a surface of a printing medium with a dot recording head. The dot recording apparatus comprises: a dot-forming element array arranged on the dot recording head to face the printing medium, the dot-forming element array comprising a plurality of dot-forming elements for forming a plurality of dots of an identical color at a substantially constant pitch in a sub-scanning direction; a main scan driving unit that drives at least one of the dot recording head and the printing medium to carry out main scan; a head driving unit that drives at least part of the plurality of dot-forming elements to form dots in the course of the main scan; a sub-scan driving unit that drives at least one of the dot recording head and the printing medium every time when the main scan is completed, thereby carrying out sub-scan; and a control unit that controls the above units.
The control unit comprises: a recording mode storage unit that stores a plurality of dot recording modes having a substantially equal recording speed, as alternative dot recording modes each defining operations of the main scan and the sub-scan for recording dots at an identical recording resolution; a mode selection information setting unit, in which mode selection information is set to specify a desired dot recording mode among the plurality of dot recording modes; and a unit that executes the dot recording according to the desired dot recording mode specified by the mode selection information.
The above dot recording apparatus enables a desired dot recording mode for attaining high image quality to be selected among a plurality of dot recording modes stored in the recording mode storage unit and set in the mode selection information setting unit in each individual dot recording apparatus. This enables adoption of a desired dot recording scheme suitable for each individual dot recording apparatus accordingly.
The plurality of dot recording modes may be different from each other in at least one of two scanning conditions of: a sequence of sub-scan feed amounts and dot-forming elements used for each main scan.
In this arrangement, the plurality of dot recording modes are different from each other in combinations of the dot-forming elements used for recording the adjoining main scanning lines. In the case where mismatching of distances between the recorded main scanning lines causes deterioration of the image quality, this arrangement enables selection of the dot recording mode to minimize the deterioration of the image quality.
The mode selection information is preferably set in a rewritable non-volatile memory. In this arrangement, the mode selection information can be stored in each individual dot recording apparatus and can be rewritten if required.
Preferably, the plurality of dot recording modes include a first type of dot recording mode, in which a number of effective dot-forming elements and an element pitch are set at integers of at least two that are not relatively prime, and in which a plurality of different values are used in combination for sub-scan feed amounts for a plurality of sub-scans, the number of effective dot-forming elements representing a net number of raster lines in a main scanning direction to be recorded in one main scan, the element pitch representing a pitch of the plurality of dot-forming elements expressed in unit of a number of dots. Use of the first type of dot recording mode ascertains a large number of equivalent dot recording modes that have an identical resolution and an identical recording speed but adopt different combinations of sub-scan feed amounts. Selection of the desired mode suitable for each individual dot recording apparatus among the large number of equivalent dot recording modes enables an image of higher image quality to be recorded.
Preferably, the plurality of dot recording modes further include a second type of dot recording mode in which the number of effective dot-forming elements and the element pitch are set at integers that are relatively prime. This arrangement enables a variety of different values to be set to the number of the dot-forming elements. This accordingly improves the flexibility in selection of a desired mode from the viewpoint of image quality and recording speed for each individual dot recording apparatus.
The plurality of dot recording modes may include a dot recording mode in which dot-forming elements used in each main scan are selected for each main scan. This arrangement increases the degree of freedom in selecting a dot recording mode and thereby enables selection of the dot recording mode for attaining higher image quality.
The plurality of dot recording modes may include a plurality of dot recording modes that have a substantially equal number of effective dot-forming elements representing a net number of raster lines in a main scanning direction recorded by one main scan; and one of the plurality of dot recording modes having the identical recording resolution and the substantially equal number of effective dot-forming elements may be specified by the mode selection information. This arrangement also increases the degree of freedom in setting the dot recording mode and thereby enables selection of the dot recording mode for attaining higher image quality.
A plurality of recorded test images, which are obtained by recording a specific test image in the plurality of dot recording modes, may be captured by a sensor, respectively; and outputs of the sensor may be analyzed to thereby select the desired dot recording mode among the plurality of dot recording modes. This arrangement enables selection of a dot recording mode for attaining high image quality by analyzing the identical test images.
The test image may include a plurality of straight lines that are recorded by different dot-forming elements and extend in a main scanning direction. In this case, the selection of the desired dot recording mode may be carried out by determining an evaluation value which represents a scatter of intervals of the plurality of straight lines for each of the plurality of dot recording modes, and selecting the desired dot recording mode based on the evaluation value. Since the evaluation value represents a scatter of distances between raster lines, a dot recording mode for attaining high image quality can be determined readily by utilizing this evaluation value.
The present invention is also directed to a method of recording dots on a surface of a printing medium with a dot recording head while carrying out a main scan in a direction substantially perpendicular to a sub-scanning direction, the dot recording head having a dot-forming element array including a plurality of dot-forming elements for forming a plurality of dots of an identical color arranged at a substantially constant pitch in the sub-scanning direction. The method comprises the steps of: (a) recording a specific test image in a plurality of dot recording modes that have a substantially equal recording speed, the plurality of dot recording modes defining operations of the main scan and the sub-scan for recording dots at an identical recording resolution; (b) selecting among the plurality of dot recording modes a desired dot recording mode corresponding to a desired recorded test image which has been selected out of a plurality of recorded test images recorded in the plurality of dot recording modes; (c) setting mode selection information for specifying the desired dot recording mode; and (d) recording dots according to the desired dot recording mode specified by the mode selection information.
This method also enables adoption of a desired dot recording scheme suitable for each individual dot recording apparatus to record high quality images.
The present invention is further directed to a computer program product storing a computer program for causing a printing apparatus to record dots on a surface of a printing medium while carrying out a main scan in a direction substantially perpendicular to a sub-scanning direction, the printing apparatus comprising a dot recording head having a dot-forming element array including a plurality of dot-forming elements for forming a plurality of dots of an identical color arranged at a substantially constant pitch in the sub-scanning direction. The computer program product comprises: a computer readable medium; and a computer program stored on the computer readable medium. The computer program comprises: a first program code that causes a computer to read a desired dot recording mode from a first storage unit for storing a plurality of dot recording modes according to mode selection information which has been set in advance for specifying the desired dot recording mode among the plurality of dot recording modes; and a second program code that causes the computer to execute dot recording according to the desired dot recording mode specified by the mode selection information.
When the computer program is executed by a computer, a desired dot recording scheme suitable for each individual dot recording apparatus can be adopted to record high quality images.
Various medium can be used as the computer readable medium such as: flexible disks, CD-ROMs, magneto-optical disks, punch cards, printed matters on which codes such as bar codes are printed, internal storage devices (memories such as RAMs and ROMs), and external storage devices.